1. Field of the Invention
The present invention relates to a cathode for a lithium secondary battery, and more particularly it relates to a sheet-type cathode and a production method for the same, and a lithium secondary battery utilizing the sheet-type cathode.
2. Description of Related Art
Lithiated cobalt dioxide and lithiated nickel dioxide are compounds having a so-called .alpha.-NaFeO.sup.2 type structure in which lithium ions are regularly arranged in a layered manner alternately with cobalt ions or nickel ions vertically to the closest packing layer of oxygen ions. Due to this structure, the lithium ions in a layer can be diffused comparatively easily. Lithium ions can be electrochemically doped or undoped in such a compounds Various studies have been made to apply these compounds to a cathode material in a lithium secondary battery that is expected to play an important role as a high-performance compact secondary battery, and in the futures as a power supply for an electric vehicle or a power storage device for load leveling.
Lithiated cobalt dioxide is already used as a material for a cathode in lithium secondary batteries that supply power to some portable telephones and video cameras. in consideration of material cost, however, lithiated cobalt dioxide, which is produced from an expensive cobalt compound, is inferior to lithiated nickel dioxide, which can be produced from an inexpensive and abundant nickel compound.
The charging/discharging characteristic of lithiated nickel dioxide, however, largely depends upon its synthesis method. It is difficult to synthesize lithiated nickel dioxide having a large discharge capacity compared to lithiated cobalt dioxide. The reason for this difficulty is that in lithiated nickel dioxide, nickel is easily substituted at a lithium site, and therefore, the resultant compound can contain substituted nickel at the lithium sites unless an appropriate synthesis condition is selected. Nickel at the lithium sites inhibits the diffusion of lithium ions, thereby adversely affecting the charging/discharging characteristic of the resultant lithiated nickel dioxide
Recently, various attempts to synthesize lithiated nickel dioxide having a large discharge capacity have been made by optimizing the synthesis conditions. For example, Japanese Laid-Open Patent Publication No. 5-290851 discloses one of the methods for synthesizing lithiated nickel dioxide. In this method, lithium hydroxide and nickel hydroxide are mixed at a proportion of 1 mole of nickel hydroxide to 1.15 to 1.75 mole of lithium hydroxide, and the mixture obtained is heated at a temperature of 450 to 900.degree. C. in air, thereby giving Li.sup.x NiO.sup.y (wherein 1.15.ltoreq..times..ltoreq.1.75 and y &gt;0). The publication describes that Li.sup.x WiO.sup.y obtained by this method exhibits a large discharge capacity of 150 mAh/g or more.
European Laid-Open Patent Publication No. 573040 discloses another method for synthesizing lithiated nickel dioxide. In this method, 1 mole of at least one nickel compound selected from the group consisting of nickel carbonate, nickel nitrate, nickel hydroxide and nickel oxyhydroxide and more than 1 mole of at least one lithium compound selected from the group consisting of lithium nitrate, lithium carbonate and lithium hydroxide are mixed so as to achieve a composition of 1&lt;Li/Ni.ltoreq.1.4. The mixture obtained is fired to give lithiated nickel dioxide having a large discharge capacity.
A liquid or solid electrolyte is generally used in a lithium secondary battery, and such an electrolyte has a smaller conductivity than an aqueous type electrolyte used in a nickel-cadmium battery or a nickel-hydrogen battery. Therefore, in order to minimize the internal resistance and maximize an obtainable current value, cathodes and anodes are generally formed in the shape of a sheet, so as to increase the area of the electrodes. Such sheet-type electrodes are laminated or wound with a separator interposed therebetween in the production of a lithium secondary battery. Therefore, the production procedure includes a process to form a sheet-type electrode from an active material.
However, when powder including lithiated nickel dioxide having a large discharge capacity is used as an active material for a cathode, the density of the sheet-type cathode tends to be lowered in the cathode forming process where the active material, a conductive substance, a binder and a solvent are kneaded and the thus obtained paste is coated on a sheet-type current collector. Owing to the low density of the cathode, the amount of the active material to be charged in a battery container with a limited volume is decreased. As a result, the resultant lithium secondary battery cannot attain sufficient performance.